The Swedish patent application No. 8003776-5 discloses a foam composite material, which includes a web of woven or non-woven material. The web is impregnated with a curable resin and contains expanded microspheres. The curable resin in the material is only partly cured and there is thus a remaining capability of bonding. Although this foam composite material can be used as such, it is particularly suitable for use in the production of different kinds of laminates, both by lamination of several such foam composite materials with other materials. The expanded microspheres confer a low density to the laminates, while the fibre web gives a high strength and local weaknesses are avoided by a homogeneous distribution of the microspheres in the fibre material. The fibre web is also a carrier for the microspheres and facilitates a uniform pick-up and distribution of these and is also of importance in obtaining a uniform heat transfer at the expansion of the microspheres. In addition to a low density, the microspheres also give the foam composite material flexibility and compressibility. These properties allow an adaption of the material to irregularities in other materials in connection with the lamination, and this generally improves the adhesion and also acts to smooth out the surface. The compressibility also means, that it is possible to vary the final density of the laminate by means of the press operation. The only partially cured resin in the foam composite material means, that many different materials can easily be bonded to the foam composite material and the resin is still sufficiently soft at the lamination operation, so that all the above-mentioned flexibility properties can be made use of. The final product, with the cured resin, has good dimensional stability, a high stiffness relative to the weight and is not brittle.
The foam composite material has been prepared by impregnating a fibre web with a liquid containing the curable resin and unexpanded microspheres and the web is then heated to expand the microspheres, to partly cure the resin and to dry the web. It has, however, been found, that this method of production causes problems in some cases. The application of heat at the expansion can sometimes cause an uncontrolled curing of the resin, which is present at the same time. Dense and compact fibre webs cannot be impregnated even with unexpanded microspheres, so that a homogeneous distribution is obtained. If, in order to avoid these problems, the liquid instead is added at the formation of the web, the compacting of this is made more difficult, and there will be problems with operation shut-downs due to the presence of the resin in the equipment for formation of the dense fibre web. Corresponding apparatus problem may arise at any desired post-treatment of the impregnated web. Since the resin will first wet unexpanded microspheres and then be deposited on the fibres, there will often be a too small amount of resin on the microspheres after the subsequent expansion, due to the considerable increase of the surface of the spheres at the expansion, which gives poor adhesion. The problems are particularly pronounced, when the fibres are more hydrophilic than the microspheres, e.g. when cellulose fibres are used.